Method and apparatus for balancing subsea internal and external well pressures

ABSTRACT

Method and apparatus for maintaining a pressure balance between internal and external subsea well pressures during underwater drilling, entry and reentry operations conducted from a floating vessel remote from the subsea well comprising injecting gas into the wellhead apparatus in amounts sufficient to cause the density of the well fluid or mud returns below the surface of the sea to approximate the density of sea water and controlling the injection of such gas and thus the internal well fluid pressures by sea water well pressure differential and control means associated with valve means located in gas injection and return lines at the subsea well apparatus.

XR 39603a409 United States Patent [72] Inventor Bruce J. Watkins PalosVerdes Estates, Calif.

(21] Appl. No. 811,052

[22] Filed Mar. 27, 1969 [45] Patented Sept. 7, 1971 [73] Assignee ReganForge and Engineering Company San Pedro, Calif.

[54] METHOD AND APPARATUS FOR BALANCING SUBSEA INTERNAL AND EXTERNALWELL PRESSURES 8 Claims, 7 Drawing Figs.

[51] 1nt.Cl ..E21b15/02,

E2lb 41/00 [50] Field of Search 175/7, 8,

Primary Examiner-Stephen .l. Novosad Attorney-Miketta, Glenny, Poms &Smith ABSTRACT: Method and apparatus for maintaining a pressure balancebetween internal and external subsea well pressures during underwaterdrilling, entry and reentry operations conducted from a floating vesselremote from the subsea well comprising injecting gas into the wellheadapparatus in amounts sufficient to cause the density of the well fluidor mud returns below the surface of the sea to approximate the densityof sea water and controlling the injection of such gas and thus theinternal well fluid pressures by sea water well pressure differentialand control means associated with valve means located in gas injectionand return lines at the subsea well apparatus.

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PATENTEDSEP Han 3.603409 SHEET 3 0F 3 EruoD AND APPARATUSFORBALANCINGLSUBSEA .flNTERNAL-hND-ExTERNAL WELLPRESSURES ,BAGKGRQUND OFTHE INVENTION Heretoforemost subsea well drilling operations haveutilized a largediarneter riser or'conduit extending from a floatingvessels drilling gig down to the .ocean floor where awell is to be'drilledh'llhe riser orlarge diameter conduitYhas-been ,used

to guide the drill string andiassociated, drill-bit ,into blowoutpreventers associated .with the wellheadas well as to provide a returnpath-for. drilling .fluids. or .m ud being circulated betweenthefloating vessel and the ,well duringjdrilling operationsaln subseadrilling; particularly in deep water locations,

the use of. such'large .diameterrisers becomes impractical because .ofthe high stresses imposed on the riser by surface and subsea watercurrents, weight of the drilling fluids or mud and,uncontrolled-movement of the floating vessel relative .to the subseawell.3 In order to attempt to overcome these difflculftiesithasbeencommon heretofore to .try to maintain the riser intension between thevessel and subsea-well by employing expansive cumbersome devices-.,which have not proved entirely satisfactory.

ln deep jwatersubsea locations, it is desirable to eliminate the-expensive and cumbersome 'devices'referred to above.

topfjofi the'blowout preventer stack-provided at the wellhead.

,However,; the substitution of arrotating blowout preventer at m t s ofthewellhead apparatus fortheaforementioned riser .creates at leasttwomajor problemsJThewell'fluid or mud being more dense thanflsea waterin the return lines creates a back pressure inside thev blowoutpreventer stack, such pressure. frequently being. twice the amount ofpressure of the surrounding-sea water. This means thatsuch-rotatingblowout :prevcntets, in deepsubsea well installations must operate con--,tinuously;at'veryhigh pressure-differentials between that of the;internal well fluids, and the a surrounding sea .water. Such pressuredifferentials increase the wear on the blowout preventer sealsand theassociated bearings. Also, there are elements} inf the drill; stringsuchas tool bitsand stabilizers, as .wen; as. other tools,-.that cannotbe; easily stripped through the rotlating blowo,ut.preyenterwhen itisnecessary to, remove themfrom the well. It has not beenpossible-heretofore to merelyopenthe wellhead upyand removesuchtoolswithout .losingall of; the drillir g fluids ormud below sea. level. Ithas heretofore been necessary to-provide a plurality of blowoutpresenters spaced a. s utficientv distance apart so that oddshapeddrilling,toolsl anbc pulled-into a space between theblowoutpreventers with the upper one closed while the lower is opened.The lower blowout preventer or-preventers are then .closedand-the upperone or more opened inorder to allow removalof the tools. Such anarrangement requires an increase in the-size, and cost of the subsea.equipmentand also creates serious, instrumentation problems, i.e.manipulating thevarious tools and controls for operating the blowoutpreventersfrom the remote floatingvessel.

It is therefore the primary object of the present invention to solve theabove problems by providing a method and apparatus for equalizing orclosely balancing the drilling fluid pressure inside the wellhead withthe surrounding sea water pressure sothat the wear on the rotatingblowout preventer is reduced and the underwater well can be opened forremoval and reentry. of tools without loss at drilling fluid.

It isa further objectof the present invention to provide methods andapparatus for equalizing such well apparatus internal and externalpressures by injecting a fluid of lower density than water, suchas-gas,into the well fluids at the wellhead apparatus prior to their-retumthrough'the mud return lines to reduce thedensity of the drillingfluidsto approximately that of the surrounding seawater.

' provided for applying aback pressure on the well fluids in thewellhead apparatus to increase the internal wellhead pressures when theyfall below a desired level.

SUMMARY OF THE INVENTION -Generally stated, the method of maintaining apressure balance between internal and external subseawell pressures at asubsea well apparatus installation-during drilling, entry and reentryoperations conductedfrom'a remote floating vessel of ..the presentinvention comprisesinjecting gas, such as air, into the subsea well inamounts sufficient to cause the density of the well fluids below thesurface of the sea to approximate the density of sea watenMoreparticularly, two or more fluid flow lines are established between thefloating vessel and the bore of the subsea well apparatuswith fluidreturns from the well being returned through one of the lines. Gas isinjected into the bore of the subsea well through the other of saidlines in amounts sufficient to lower the density of the drilling fluidsin the return line or lines thus exerting a pressure uponthe internalbore of the well apparatus approximately equal to the pressure exertedthereon by the column of sea water above the apparatus. When suchbalance is achieved, the underwater well vmay be opened for the removaland reentry of tools without theloss of drilling muds or other wellfluids. i

Theapparatus of the present invention, in general, includes one or moregas supplylines for injecting gas, such asair, into the subsea wellapparatus borefor mixing therein with the drilling fluids or mud beingreturned to thesurface fluid return line or lines also provided. Valvemeansare provided in the gas supply line or lines at a locationgenerally adjacent the subsea-well for controlling the supply of gas tothe well bore.

As further contemplated within the present invention, sea

water well pressure'differential sensing means are provided inassociation with the valve means for operating such valye means inresponse to a pressure differential between that within the subsea wellapparatus bore and that of the surrounding sea water. Also, it iscontemplated that similar valve means and pressure differential sensingmeans may be utilized in association with the drilling fluid or mudreturn line or lines for applying aback pressure within the wellapparatus bore through the return line or lines in the event internalwell pressures fallbelow that of thesurrounding sea water.

Further objects, various advantages and a better understanding of theapparatus and methods of the present invention will become apparent tothose skilled in the art'from a consideration of the following detailedexplanation of an exemplary embodiment thereof. Reference will be madeto the appended sheets of drawings in which:

IN THE DRAWINGS FIG. 3 is a somewhat schematic representation of thesubsea equipment of FIG. 2, taken partially in section, illustrating theinjection of gas into the well bore and drilling mud being returned upthemud return line;

FIG. 4 is another-somewhat schematic view of the equipment of FIG. 2showing the drill string removed, the well open to the surrounding seawater and illustrating how back pressure through the mud return line canbe utilized in the balanced pressure drilling system of the exemplaryembodiment of FIGS. 1 through 4.

FIG. 5 is a detail view of a portion of the equipment of FIGS. 1 through4 partially in section, showing the associated valve apparatus in aclosed position;

FIG. 6 is a plan view of a portion of the equipment of FIG. takentherein along the plane Vl-Vl; and

FIG. 7 is another section view of the equipment of FIG. 5 showing thevalve apparatus in an open position.

The method and apparatus for a balanced pressure drilling system willnow be described in detail with reference to the aforedescribed figures.Referring to FIG. I initially, a floating vessel or barge 10 ispositioned by suitable anchoring means (not shown) in a body of water orsea 11 over a subsea well formation 12. A conventional drilling rig,indicated at 13, may be provided on the barge or vessel 10 for running aconventional drill string 14 down to the subsea equipment, indicatedgenerally at 15, at the well site.

In accordance with the method and apparatus of the present invention oneor more gas supply lines, as line 16, and one or more well fluid ordrilling mud return lines, as return line 17, are connected in a mannerhereafter described in greater detail between vessel 10 and the subseaequipment indicated generally at 15. A compressor, or other source, ofgas or air on vessel 10 may be utilized for injecting or introducing gasinto the well fluids or drilling mud being returned up line 17 in orderto control the density thereof. Preferably, the gas or air introducedinto the well fluids or drilling mud at the subsea equipment via supplyline 16 reduces the density of the returns to that approximating thesurrounding sea water. Thus, internal pressures within the subseaequipment due to the weight of the well fluid, or mud, returned in line17 can be balanced against the external well pressures due to thepresence of the surrounding sea water.

Referring now to FIG. 2, it can be seen that the drill string 14 isprovided in conventional manner with a drill bit 18 at a lower end fordrilling a well hole 19 through the well equipment indicated generallyat 15. In the preferred exemplary embodiment, the well equipmentindicated generally at comprises a stack of blowout preventers andrelated equipment mounted upon drilling template 20 positioned over thelocation for drilling well hole 19. The stack of blowout preventers 21,22 and 23 are mounted by a connector 24 upon template 20. A rotatingblowout preventer 25 is mounted to the top of the aforementioned BOPstack by connector 26. Rotating BOP 25 is illustrated with conventionalflanged guides 27, 27' mounted upon guide arms 28, 28 for running therotating BOP to the subsea equipment via the preconnected lines 16 and17.

The drill string may be run in conventional manner in cooperation withthe bit guide 29, the latter having flanged guides 30, 30' mounted onthe guide arms 31, 31, respectively. Choke and kill line-type valves 32and 33 may be provided in the connections 34 and 35, respectively,between the subsea equipment inner bore and the lower ends of the flowline 16 and 17. Hydraulically or manually operated valve actuators 36and 37 may also be provided in known manner. Valves 32 and 33 may beutilized for conventional choke and kill operations unrelated to thebalanced pressure drilling method and apparatus of the presentinvention.

As particularly contemplated within the present invention, the internaland external subsea well pressures are balanced during drilling, entryand reentry operations conducted from the floating vessel 10. Suchbalancing of the internal and external well pressures is accomplished byintroducing gas, such as air, into the subsea well bore via gas supplyline 16 to control the density of the well fluid, such as drilling mud,returns back through return line 17. In accordance with the method andapparatus of the present invention, valve means are introduced into thegas supply line and are controlled by pressure differential sensingmeans which sense a pressure differential between the internal well borepressure and the external, surrounding sea water pressure. In thepreferred exemplary embodiment, such valve means and associated pressuredifferential sensing means are indicated generally at 40. Similar valveand sensing means, indicated generally at 50, as also contemplatedwithin the present invention are provided between the well bore andreturn line 17.

Referring now to FIG. 3, the subsea equipment of FIG. 2 is illustratedpartially in section to somewhat schematically show the introduction ofgas, such as air, into the well fluids, such as drilling mud, beingreturned from the well bore up through the return line 17. During aconventional drilling operation, mud is introduced down through thedrill string 14, through the drill bit 18 and then circulates back upthrough the annulus 38 formed between the well bore 19 and the exteriorsurface of drill string 14. Normally, such drilling fluids or mud havebeen returned up through the return line 17 to the vessel, with theweight of the dense drilling mud creating considering high pressureswithin the well bore below the rotating blowout preventer 25. Therotating blowout preventer is thus required to operate under conditionsof continuous high differential pressure which is damaging to both theseals and bearings thereof. Further, there are frequently elements inthe drill string, such as bits and stabilizers, that cannot be strippedup through the rotating blowout preventer when it is necessary to removethem from the well.

In accordance with the present invention, these problems are solved bythe equalization of the pressure inside the wellhead with thesurrounding water pressure by controlled introduction of gas or air intothe mud returns via the valve and sensing means indicated generally at40. As seen in FIG. 3, gas is injected down line 16, through valve andsensing means 40 into the well bore 38 where it comingles with the wellfluid returns 39. The gas injection into the fluid returns 39 lowers thedensity thereof and the mixture of gas and returns in line 17. Bycontrolling the amount of gas injected through means 40, the density ofreturns in line 17 may be controlled to maintain the pressuredifferential across the rotating blowout preventer 25, and thedifferential between internal and external well pressures, at a minimum.

Referring to FIG. 5, the exemplary embodiment of valve means anddifferential pressure sensing means are illustrated generally at 40connected between the gas supply lines 16 and the well apparatus(blowout preventer 21 in the exemplary embodiment). The valve means ofthe exemplary embodiment includes the valve body 41 slidably mountedwithin the valve housing 47. Piston member 42, connected to valve body41 by stem portion 43, functions to stabilize the sliding movement ofbody 41 in bore 46 and as described subsequently, forms a part of thepressure sensing means. Bodies 41 and 42 are provided with fluid sealsor O-ring seals 44 and 45, respectively, for sealing the bodies ormembers 41 and 42 within bore 46 of valve housing 47. Bore 46 isenlarged in bore area 48 adjacent the gas inlet conduit 16 and area 49adjacent the outlet conduit 38 which is in fluid communication with thewell apparatus bore 38.

Valve body or member 41 is shown in closed position in FIG. 5. Gas flowfrom line 16 into the well bore via line 38' is prevented due to thesealing engagement of seal means 44 and the surrounding housing bore 46.However, on an upward movement of valve member 41 into the open positionof FIG. 7, gas flow from line 16', through housing bore 46 and line 38to the well apparatus bore 38 is allowed. The opening and closing ofvalve member 41 relative to the lines 16' and 38' is controlled byassociated pressure differential sensing means as hereinafter described.

Pressure differential sensing and valve actuating means are provided inassociation with the valve member 41 for operating such valve inresponse to a pressure differential between the surrounding sea waterand the well apparatus bore pressure. In the exemplary embodiment, suchpressure sensing means are provided by the upper and lower members 41and 42 which act as piston members in housing bore 46. As seen in FIG.5, valve member 41 is provided with a round pistonlike configurationwith an upper end fluid seal 51. The upper end of housing 47 is open tothe surrounding sea water pressure through the open end of bore 46. Inthe exemplary embodiment, an end fitting 52 is shown bolted by bolts 53onto the upper end of housing 47 with a central threaded aperture orbore 54. While in the preferred exemplary embodiment, the

The lowerend-of h'ousing 47- is also providedwithan' end fitting 55boltedzto the housing. A conduit56"isconnected'to the valve housing bore46 via-fitti'ng 57, threaded into end fittingaSSi' The other'end of.conduit 56 is connected'into thelines 38-on 'th'e welliapparatus boreside. ofthevalve housing.

Well apparatus: borepressures in here 38", on-'thej well apparatus'side:ofhousingfiflare thus communicated via-conduit 56" form: line 38 tothe bottom end of thesecond valve. memberbrpistonA l ReferringrtdFlGk 7;wellapparatus\bore 38 exceed the surrounding sea-water pressure, suchpressure'diffrential betweenf the. upper-and lower ends: of-integralmembers 4l, 42 an'd'43 'causesan upward movement ofsuch members-fromthepositionnof FIG. 5 to that of 'FlG. 73' Gas :is thenjinjectedthrough'line-l6 valve housing- 47 and'line' 38 intotherwell'apparatusbore until the variation of'the wellfluidreducesathe well apparatus borepressures to a value equal to or slightly less than the surroundingsea'rwater' pressures. Whenthe sea water pressuresexceed that'of'thecinternal apparatuspressures, valve and piston members4l and 4-2aremoved"back toj the-closed positi'onbfFlGl 5 stopping furtherinjectionof'gas into the well fluidreturns;

ltis ldesiredFtomaintain well bore pressures sligh'tly'above th'evalueofthe' surrounding sea water pressures-the diameter offther'lowen piston42 may be; made somewhat smaller than thatof-th'e upper valve member orpiston 4f. However, in the preferred exemplary embodiment, the"diameters'of members 41" and-'42,anditheir corresponding-pressuresurfaces-exposed within=tore.'-46at-'the upperandlowerend's'of members41 and 42T- are equal to each other. With" this arrangement, the associated =valve' rneans and-pressure differential control means E1683:5' through 7 ca'nfi be utilized to maintain the; wellapparatus"-internal bore pressures and' surrounding; sea waterpressures-very nearlybalanced duringth'e' drilling operations.

in the event that the pressure withinthewell apparatusbore38'rfallsbelowa-thesurrounding seawater pressure, itiscontemplatedlwithin ltheipresent invention thata valve means andassociated-pressure differential'sensingand valve control 'app'aratus;indicated z generally at 50, constructed and" operated as apparatus 40;may be utilized in association with the mud retumline l'7-asillustratedin 'FlG; Si TheintemaI construction and operation ofthecombinationvvalve and pressure differential sensing and valve control meansindicated generally at50is the same-asthatfillustrated in-FlGS. 5 and 7;However, the inlet conduit l'Vto valve and pressure control means 50 isconnected into'themud return line l7 while the outlet conduit 38 isagain connecte'd into the well app'aratusbore 38; as best seemin-FIG; 43ln=theeventthe pressure-within bore 38 falls below thav of thesurrounding sea water pressure, the differential control mea'nsassociated with means 50 senses" the difference in presSurebetWeen thewell fluids within the conduit'38" and "the surrounding-seawater, opensthe associated valve-means and-allows'return'ofmud or=drilling fluidsback downline l-7Jinto thejwell; The necessary back pressure in line l7'may: be obtained'bwpumpihg,drillingfluid back down line 17 inordertomaintain'the, desired balance between internal and external wellpressures:

By maintaining a very nearbalance" between= internal and external=wellbore pressures'as'aforedescribed, the subsea apparatusmay'be openedup to=thesurrounding sea-water as shownin FIG; 4;,and tliedi'ill stringlfland associated tool, asbit 18; removed therefrom. As illustrated inFIG. 4, the blowoutpreventers are allin open position with thesurrounding sea water pressure in balance with the internal well fluidto maintain'control over the well even though it is open to thesurrounding'sea'water. It can be seen'from the foregoing disclosure'thatby usingsthe=pressurebalancing apparatus and method 'of thepresentinventiong' the instrumentation required and theproeedureszfofentryuand reentry of subsea well apwh en' the well'fluidpressures within" paratus are greatly simplified. Drillingtools may beremoved andzinserted'into the subsea welliapparatus with the samecaseand conveniencein-the subsea environmentasthoughthc wellapparatus'were on land Having thus described'an'exemplary 5 embodimentof the present. invention, what-l claim as my invention-is set forth-"inth'e-followingclaims.

[claim- 1. In a method of maintaining apressure balance between internaland external subseawell pressures during drilling, entry'and'reentryoperation conducted from afloating vessel remote fromthesubsea-wellincluding the step of injectingigas into thesubsea well inamountssufficient to cause the density of'the-iwell fluids below-thesurface of the-sea to approximate l the=density of seawater, theimprovement comprising the'additional steps of;

sensingthe pressure differential at said subsea well between intemalwellborepressureand external waterpressure and;

controlling theamount of gas injected into said well bore to minimizethe magnitude of said pressure differential beingsensed.

2. In a method of maintaining a pressure balance between internal andexternal subsea well pressures during drilling, entry and reentryoperations conducted from a floating vessel remote: from thesubseawell'wherein drill mudis circulated from the vessel'into the wellduring drilling operations-and is retumedftothevessel via a mud returnline including the steps of establishingztwo ormorefluidflowlinesbetween the float- 'ingvessel and the bore of the subsea well,returning drill mud to the floating vessel from the well bore via one ormore return lineslof said fluid 'flow'lines and injecting gas into thesubsea well through one or more injection'lines of said fluid flow linesin'amounts sufficient'tolower the density of said drill mud in 7 saidone or morev return lines to approximate the density of sea water; theimprovement comprisingthe additional stepof:

applying a back pressure on said well bore by pumping drill mud down oneor more of "said lines to balancethe internal and external wellpressures when the internal well pressure fallsbelow that'ofthesurrounding sea water.

3." The .methodof claim 2 comprising the additional step of:

sensing the pressure differential at said subsea well betweenthepressurewithin saidvone or more return lines'at said well andthesurrounding-water pressure, and

controlling the amount" of drill mud introduced back into said wellthrough said lines to minimize the magnitude of said. pressuredifferential.

4'5 The method of claim'3comprising the additional step of:

opening the well bore to the surrounding sea water to allow removal ofand re-entry of well tools when the internal andexternal well pressuresare in balance.

5. In a subsea well drilling apparatus including a drill stringanddrilling mud-retum line, each run from a floating vessel 5 5 overasubsea well location to subsea drilling equipment at the well location,a blowout'preventer'means for-sealing the well bore about the drillstring within the well during drilling operations and gas supplylinevmeans run from said floating vessel and connected into said wellbore below said blowout 0 preventer means for injecting gas into thedrilling mud to reduce its density as it is returned from said well tothe floating- 6. In a subsea well drilling'apparatus including'a drillstring and d'rillingmud return line, each run from a floatingvesseltherein adjacent said subsea well for controllingthe,

over a subsea well location to subsea drilling equipment at the welllocation, a blowout preventer means for sealing the well bore about thedrill string within the well during drilling operations and gas supplyline means run from said floating vessel and connected into said wellbore below said blowout preventer means for injecting gas into thedrilling mud to reduce its density as it is returned from said well tothe floating vessel via said mud return line, the improvement comprisingthe provision of:

valve means in said mud return line in a location therein adjacent saidsubsea well for controlling the back flow of mud from said return lineback into said well bore; and pressure differential sensing and valvecontrol means in association with said valve means for operating saidvalve 8. The method of claim 2 comprising the additional steps of:

opening said well bore to the surrounding sea water and running a welltool between said well bore and said floating vessel through said seawater and the opened well bore.

1. In a method of maintaining a pressure balance between internal andexternal subsea well pressures during drilling, entry and reentryoperation conducted from a floating vessel remote from the subsea wellincluding the step of injecting gas into the subsea well in amountssufficient to cause the density of the well fluids below the surface ofthe sea to approximate the density of sea water, the improvementcomprising the additional steps of: sensing the pressure differential atsaid subsea well between internal well bore pressure and external waterpressure and controlling the amount of gas injected into said well boreto minimize the magnitude of said pressure differential being sensed. 2.In a method of maintaining a pressure balance between internal andexternal subsea well pressures during drilling, entry and reentryoperations conducted from a floating vessel remote from the subsea wellwherein drill mud is circulated from the vessel into the well duringdrilling operations and is returned to the vessel via a mud return lineincluding the steps of establishing two or more fluId flow lines betweenthe floating vessel and the bore of the subsea well, returning drill mudto the floating vessel from the well bore via one or more return linesof said fluid flow lines and injecting gas into the subsea well throughone or more injection lines of said fluid flow lines in amountssufficient to lower the density of said drill mud in said one or morereturn lines to approximate the density of sea water, the improvementcomprising the additional step of: applying a back pressure on said wellbore by pumping drill mud down one or more of said lines to balance theinternal and external well pressures when the internal well pressurefalls below that of the surrounding sea water.
 3. The method of claim 2comprising the additional step of: sensing the pressure differential atsaid subsea well between the pressure within said one or more returnlines at said well and the surrounding water pressure, and controllingthe amount of drill mud introduced back into said well through saidlines to minimize the magnitude of said pressure differential.
 4. Themethod of claim 3 comprising the additional step of: opening the wellbore to the surrounding sea water to allow removal of and re-entry ofwell tools when the internal and external well pressures are in balance.5. In a subsea well drilling apparatus including a drill string anddrilling mud return line, each run from a floating vessel over a subseawell location to subsea drilling equipment at the well location, ablowout preventer means for sealing the well bore about the drill stringwithin the well during drilling operations and gas supply line means runfrom said floating vessel and connected into said well bore below saidblowout preventer means for injecting gas into the drilling mud toreduce its density as it is returned from said well to the floatingvessel via said mud return line, the improvement comprising theprovision of: valve means in said gas supply line means at a locationtherein adjacent said subsea well for controlling the supply of gas tosaid well bore through said supply line means and pressure differentialsensing and valve control means in association with said valve means foroperating said valve means in response to a pressure differentialbetween the pressure in said gas supply line on the well bore side ofsaid valve means and the pressure of the surrounding sea water.
 6. In asubsea well drilling apparatus including a drill string and drilling mudreturn line, each run from a floating vessel over a subsea well locationto subsea drilling equipment at the well location, a blowout preventermeans for sealing the well bore about the drill string within the wellduring drilling operations and gas supply line means run from saidfloating vessel and connected into said well bore below said blowoutpreventer means for injecting gas into the drilling mud to reduce itsdensity as it is returned from said well to the floating vessel via saidmud return line, the improvement comprising the provision of: valvemeans in said mud return line in a location therein adjacent said subseawell for controlling the back flow of mud from said return line backinto said well bore; and pressure differential sensing and valve controlmeans in association with said valve means for operating said valvemeans in response to a pressure differential between the pressure insaid mud return line on the well bore side of said valve means and thesurrounding sea water pressure.
 7. The method of claim 1 comprising theadditional steps of: opening said well bore to the surrounding sea waterand running a well tool between said well bore and said floating vesselthrough said sea water and the opened well bore.
 8. The method of claim2 comprising the additional steps of: opening said well bore to thesurrounding sea water and running a well tool between said well bore andsaid floating vessel through said sea water and the opened well bore. >